J. clin. Path., 1975, 28, 18-24

The morphological identification of pathogenic yeasts using carbohydrate media K. R. JOSHI, D. A. BREMNER, D. N. PARR, AND J. B. GAVIN' From the Department of Pathology, University of Auckland, and Microbiology Laboratory, Auckland Public Hospital, New Zealand SYNOPSIS Eight isolates of C. albicans were used to determine the frequency with which germ tube formation occurred: on rice extract -Tween 80 agar, on its components, and on 1 % bactopeptone agar after three hr at 37°C; in 0*5 % aqueous solution of various carbohydrates; in various concentrations of glucose; on 0-5 and 0.1 % glucose agar and on various types of agar alone. Subsequently 250 isolates of yeast of the genera Candida, Torulopsis, Trichosporon, Cryptococcus, and Saccharomyces, which were obtained from a clinical laboratory, were spread on rice extract -Tween 80 agar and on 0-1 % glucose agar and covered with coverslips. Direct microscopic examination after incubation for three hours at 37°C demonstrated germ tube formation by all 140 isolates of C. albicans, but by none of the other yeasts. The characteristic features of the pseudomycelia of isolates of Candida and Trichosporon were evident on reexamination after a further 45 to 69 hours at room temperature (22°C). These morphological observations suggested the identity of the isolates of Torulopsis, Cryptococcus, and Saccharomyces but identified virtually all (98-2 %) of those of the genera which formed pseudomycelia. Of the latter group only four isolates required fermentation and assimilation tests to determine whether they were C. parapsilosis (1) or C. guilliermondii (3).

Candida albicans is the most common pathogenic Material and Methods yeast isolated from clinical specimens. In most laboratories it is identified by its rapid formation of GERM TUBE PRODUCTION ON RICE EXTRACT filamentous outgrowths or germ tubes when grown MEDIA in human serum (Taschdjian, Burchall, and Kozinn, Eight isolates of C. albicans known to form germ on bactopeptone agar (Joshi and Gavin, 1974) 1960). In studies of the reliability of this germ tube tubes and on rice extract -Tween 80 agar chlamydospores test and of the factors which supposedly mediate this (Difco Laboratories, Detroit, Michigan, USA) were transformation we have demonstrated that it can inoculated of the following also occur in trypticase soya broth (Joshi, Bremner, media: 1 %onto paired preparations in rice extract -Tween 80 Gavin, Herdson, and Parr, 1973) and in several medium; 1 %bactopeptone bactopeptone in rice extract medium; peptone media (Joshi, Gavin, and Bremner, 1973), 1 % bactopeptone plus 1 % -Tween 80 in 2% agar; and have suggested that a simple solid medium, 1 % rice extract -Tween medium; and 1 % bactopeptone bactopeptone in 2% agar, is most reliable (Joshi in 2 % agar. All the80media tested in this experiment and Gavin, 1974). were obtained from Difco Laboratories, Detroit, This paper describes a continuation of these USA. studies in which even simpler media were evaluated Michigan, The lines of inoculation were covered with large and methods evolved to carry out not only the germ the preparations incubated at 37°C coverslips tube test, but also the direct examination of the for three hrand which at stage they were examined for characteristic mycelial morphology of various germ tube formation. Then one preparation of each species of two genera using a single preparation for pair was kept at room temperature (22°C) and the both examinations. other at 37°C for another 45 hr at which time all plates were again examined. GERM TUBE PRODUCTION IN CARBOHYDRATE MEDIA

Address for correspondence: Dr J. B. Gavin, Department of Patho-

logy, University of Auckland, Private Bag, Auckland, New Zealand.

The germ tube test was then performed in 4, 2,

Received for publication 23 September 1974.



The morphological identification of pathogenic yeasts using carbohydrate media 1, and 05 % sterile solutions of glucose, and in 0 5 % sterile solutions of fructose, galactose, xylose, lactose, maltose, sucrose, mannitol, trehalose, dextrin, glycogen, soluble starch, cellobiose, raffinose, sorbitol, dulcitol, levulose, inulin, inositol, and arabinose. One millilitre aliquots of each of these media were inoculated with fresh cultures of the same eight isolates to provide an initial concentration of 1 x 106 organisms per millilitre. After incubation at 37°C for three hr, the percentage of organisms which had developed germ tubes in each preparation was determined from Giemsa-stained smears. In a subsequent experiment these eight isolates were cultured on 0 5 and 01 % glucose (May & Baker, Dagenham, Essex, England) in 2% Difco agar (Difco Laboratories, Detroit, Michigan, USA), and also on 2% Difco agar, 2 % Davis agar (Davis Gelatine NZ, Christchurch, New Zealand), 2% ion agar no. 2 (Oxoid Ltd, London, England), 2 % Noble agar (Difco Laboratories, Detroit, Michigan, USA), and 2 % agarose (BDH, Chemicals Ltd, Poole, England) alone. Coverslips were placed on the lines of inoculation. The plates were then examined for germ tube formation after incubation at 37°C for three hr and again after another 45 hr at 22°C.

by eight isolates of C. albicans on media containing 1 % peptone and/or various components of rice extract -Tween 80 agar are shown in table I. All isolates formed germ tubes within three hr at 37'C on all the five media, and all subsequently formed


No. with No. with Chlamydospores Germ Tubes afterAnother 45 Hr at: after 3 Hr Room 37 °C at 37 OC Temperature

1 % peptone plus rice 8 extract -Tween 80 agar 1 Y. peptone plus rice 8 extract agar 1 % peptone plus 1 % -Tween 8 80 and 2 % agar Rice extract -Tween 8 80 agar 8 1 % peptone in 2 % agar








8 -

Table I The formation ofgerm tubes and chlamydospores by eight isolates of C. albicans on media containing various components of rice extract -Tween 80 agar and 1 % peptone


601. Subsequently 140 isolates of C. albicans, and 110 isolates of other yeasts (C. tropicalis 22; C. pseudotropicalis 3; C. krusei 11; C. parapsilosis 33; C. guilliermondii 3; Torulopsis glabrata 31; Trichosporon cutaneum 2; Cryptococcus neoformans 1; Cryptococcus albidus 1; and Saccharomyces cerevisiae 3), which had been obtained from patients from Auckland Public Hospital and which had been identified according to Dolan (1971) were tested. Each isolate was assigned a number based on its order of receipt by the laboratory and was then inoculated onto paired plates of rice extract -Tween 80 and 0-1 % glucose agar medium. To avoid overinoculation a loop containing part of a colony was rubbed onto one side of the medium and then streaked to produce four horizontal and four vertical lines of inoculation which were then covered with sterile coverslips. One of the rice extract plates was kept at room temperature throughout the experiment while the other, together with the glucose agar plate, was incubated at 37°C for three hr, Fig l The mean percentage of organisms from eight then kept at room temperature (220C) thereafter. isolates of C. albicans which formed germ tubes in various These were then examined directly at magnifications 0 5 % carbohydrate media and in various concentrations glucose. of x 100-450; three hr and 48 hr after inoculation, ofG4-4 % glucose, G2-2 % glucose, GI-1 % glucose, and then daily for a further five days. G-O5%glucose, F-fructose, Ga-galactose, S-sucrose, M-maltose, T-trehalose, Gl-glycogen, D-dextrin, Results X-xylose, I-inulin, Du-dulcitol, Le-levulose, Ccellobiose, Ma-mannitol, R-raffinose, In-inositol,

The formation of


tubes and chlamydospores

So-sorbitol, A-arabinose, and St-soluble starch.

K. R. Joshi, D. A. Bremner, D. N. Parr, and J. B. Gaviln


chlamydospores after another 45 hr at room temperature on rice extract -Tween 80 agar. However, only three formed chlamydospores under the same conditions on media containing rice extract plus peptone and none did so on other media or when maintained continually at 37°C. The mean percentage of organisms with germ tubes formed by eight isolates in various aqueous carbohydrate media are shown in figure 1. This was greatest for glucose, fructose, and galactose. Sucrose, xylose, maltose, trehalose, dextrin, and glycogen were moderately effective in inducing germ tube formation in C. albicans, while the rest were either ineffective or only very weakly effective. Germ tube .i. formation was reduced as the concentration of glucose increased from 0 5 to 4%. Surprisingly, some organisms in all isolates formed germ tubes on all the agar media (table II) but only a small proportion did so on Noble agar and agarose. After another 48 hr at 22°C none of the isolates had formed chlamydospores on agarose, Fig 2 C. albicansafterthreehr at37 Con 0%10 only three of them did so on the rest of the media glucose agar showing thefo, mation of germ tubes (arr-o s). except when it included 01 0% glucose, in which case The refractile dots are blubbles in the medium. >' 800 five of eight isolates formed chlamydospores. characteristic clusters of blastospores distributed along the course of each pseudohypha (fig 4). Media No. with No. with All the isolates of C. tropicalis formed long, Germn Tubes Chlamydospores branched, thin pseudomycelia with sparse lateral alier Another alier 3 Hr blastospores (fig 5) in all three types of preparation. at 37 C 45 Hr at 22 ^ C Whereas all the isolates of C. pseuidotropicalis formed 0O5 % glucose in 2 % agar 8 3

> '+~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~I.

0 I % glucose in 2 % agar 2 % Difco agar 2 % Davis agar

2%Ionagarno.2 2 % Noble agar 2 % Agarose

8 8 8 8 81 82

5 3 3 3 3

Table II Formation of germ tubes and chiamydospores by eight isolates ofCandida albicans on variolus agar media 'Very small percentage of organisms formed germ tubes. 2Only occasional organisms formed germ tubes.

Of the 250 isolates of yeasts tested, all 140 isolates of C. albicans but none of the 110 isolates belonging to other species, formed germ tubes within three hr at 37°C both on rice extract -tween 80 medium and on 0-1 % glucose agar. A large proportion of the organisms of each isolate of C. albicans did so in each preparation (fig 2). After another 45 hr at 22°C, 131 isolates of C. albicans had formed chlamydospores on rice extract -Tween 80 medium. While 135 isolates did so on rice extract -Tween 80 medium kept continuously at 22°C, 130 formed chlamydospores on 0-1 % glucose agar kept for three hr at 37°C and then 45 hr at 22°C (fig 3). However, all the isolates of C. albicans grown on 01 0% glucose agar formed pseudohyphae with

Fig 3 C. albicans after three hr at 37 'C and then 45 hr at 22 'C on 0.10% glucose agar showing pseudomycelium with chlamydospores (arrows). o 1000


The morphological identification ofpathogenic yeasts using carbohydrate media


Fig 4 Pseudohyphae C. albicans on 0 1% glucose agar showing clusters ofblastospores (large arrows) and a terminal chlamydospore (small arrow). x 800

Fig 6 Pseudohyphae of C. pseudotrcpicalis on 0-1% glucose agar consist of lines of elongated cells with ovoid blastospores (arrows) lying alongside. x 800

pseudohyphae consisting of lines of spindle cells with similar elongated cells lying parallel to and alongside them (fig 6), the isolates of C. krusei all formed pseudohyphae with short lateral branches which had small terminal clusters of elongated blastospores with rounded ends giving them a 'match-stick' appearance (fig 7).

Of the 33 isolates of C. parapsilosis, 29 (87-9%) formed a typical spidery, branched, tree-like mycelium radiating from each colony on rice extract -Tween 80 agar when kept continuously at room temperature (fig 8), 25 (75-8%) did so on the same medium when kept at 37°C for three hr and then at room temperature for 45 hr, but the other isolates



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The morphological identification of pathogenic yeasts using carbohydrate media.

Eight isolates of C. albicans were used to determine the frequency with which germ tube formation occurred: on rice extract -Tween 80 agar, on its com...
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